A web crawler is a computer program that visits a webpage and scans that webpage for visible text, data, keywords, hyperlinks, and the content of the various tags used in the webpage, such as keyword rich meta tags. Each of the webpages to be crawled may be accessible via a particular reference link, such as a uniform resource locator (URL) or uniform resource identifier (URI). Entire websites or specific pages thereof can be selectively visited and indexed by a web crawler. Alternative names for a web crawler include web spider, web robot, bot, crawler, and automatic indexer.